1) Field of the Invention
The present invention relates to an apparatus for and a method of controlling a drag roller in a web type electronic printing machine, and more particularly, to a drag roller control apparatus and a drag roller control method which can control the traveling state (e.g., tension, elongation, etc.) of a web being fed.
2) Description of the Related Art
FIG. 5 shows a diagram used to explain how a web in a general web type electronic printing machine is fed.
As shown in the diagram, the web type electronic printing machine is constituted by a web feeding unit 4, a preheating roller 12, a plurality of sets of printing units 7a.about.7d, a paper feeding roller 1, a paper discharging roller 3, an intermediate drag roller 2 interposed between the printing units 7b and 7c, and a plurality of guide rollers 13 for serially guiding a web 6 to these units. Note in FIG. 5 that reference numerals 14a and 14b denote fixing rollers.
While FIG. 5 shows the web type electronic printing machine for performing 4-color printing on the obverse of the web 6, it may be a printing machine further provided with a plurality of sets of printing units. Also, it may be a duplex multicolor printing machine. That is, between the printing units, for example, between the printing units 7b and 7c, a web inverting mechanism is disposed and constructed so that the obverse and reverse of the web 6 are inverted. After 2-color printing has been performed on the obverse side of the web 6, 2-color printing is performed on the reverse side.
In the above-mentioned construction, the web 6, unwound from a rolled web 5 set in the web feeding unit 4, is moderately heated as it travels around the preheating roller 12. Then, the web 6 is fed to the first and second printing units 7a and 7b via the paper feeding roller 1. Next, after toner for 2-color printing has been attached to the obverse side of the web 6 with the first and second printing units 7a and 7b, the web 6 is further transferred to the fixing roller 14a. With this fixing roller 14a, the printing toner is fixed to the obverse of the web 6, whereby 2-color printing is performed on the obverse side of the web 6.
The web 6 on which 2-color printing has thus been performed is fed to the third and fourth printing units 7c and 7d via the intermediate drag roller 2. Then, after different toner for 2-color printing has further been attached to the obverse side of the web 6 with the third and fourth printing units 7c and 7d, the web 6 is transferred to the second fixing roller 14b. With this fixing roller 14b, the printing toner is fixed to the obverse of the web 6, whereby 4-color printing is performed on the obverse side of the web 6.
Next, after the aforementioned printing of the web 6 has been completed, the web 6 is subsequently sent out to the processes on the downstream side, in which various processing, such as cutting, folding and the like, are performed.
Incidentally, such a conventional web type electronic printing machine adopts a method of controlling the rotational speed N of the paper feeding roller 1, intermediate drag roller 2, and paper discharging roller 3 with a high degree of accuracy and adjusting the amount that the web is fed to the printing units 7a.about.7d and the amount that the web is delivered from the printing units 7a.about.7d, in order to suitably control the traveling state (e.g., tension, elongation, etc.) of the web 6 that is fed to the above-mentioned printing units 7a.about.7d.
This method, however, has the following problems.
FIG. 6 is a diagram for explaining the problems of the conventional web type electronic printing machine. As shown in this diagram, the temperature of the intermediate drag roller 2 and paper discharging roller 3 is nearly the same as the room temperature (normal temperature) before or at the beginning of start of operation. However, after start of operation, the intermediate drag roller 2 and the paper discharging roller 3 gradually increase in temperature, because they are contacted by the web 6 heated by the fixing rollers 14a and 14b. Because of this, the intermediate drag roller 2 and the paper discharging roller 3 expand, and the respective diameters increase as shown by reference numeral 2' (3') in FIG. 6.
Similarly, the paper feeding roller 1 gradually rise in temperature after start of operation, because it is contacted by the web 6 heated by the preheating roller 12. Because of this, the paper feeding roller 1 expands and its diameter increases.
With reference to this, a more detailed description will be made. As shown in FIG. 6, assume that the initial diameter of each of the drag rollers 1, 2, and 3 is D.sub.0. Also, the diameter after expansion of each of the drag rollers 1, 2, and 3 after start of operation is assumed to be D (D.sub.0 &lt;D). Furthermore, the rotational speed N of each of the drag rollers 1, 2, and 3 is assumed to be constant. At this time, the initial circumferential speed v.sub.0 of each of the drag rollers 1, 2, and 3, the circumferential speed v (v.sub.0 &lt;v) after expansion of each of the drag rollers 1, 2, and 3 after start of operation, and a difference (difference in circumferential speed) .DELTA.v between the initial circumferential speed v.sub.0 and the after-expansion circumferential speed v are expressed by the following equations (1), (2), and (3):
Initial circumferential speed: EQU v.sub.0 =.pi.D.sub.0 N (1)
After-expansion circumferential speed: EQU v=.pi.DN (2)
Difference in circumferential speed: EQU .DELTA.v=v-v.sub.0 =.pi.(D-D.sub.0).multidot.N (3)
Thus, if the respective temperatures of the drag rollers 1, 2, and 3 increase, the circumferential speeds of the drag rollers 1, 2, and 3 will increase. Therefore, the respective quantities that the web is transferred by the drag rollers 1, 2, and 3 increase, as shown in FIG. 6.
Here, the initial web transfer quantity S.sub.0 and the web transfer quantity S (S.sub.0 &lt;S) after expansion can be expressed with time as t by the following equations (4) and (5):
Initial web transfer quantity: EQU S.sub.0 =v.sub.0.multidot.t (4)
Web transfer quantity after expansion:
S=v.multidot.t (5)
Thus, if the respective quantities that the web is transferred by the drag rollers 1, 2, and 3 are increased, the traveling state (i.e., the state of the tension, deformation of elongation, etc. of the web 6) of the web 6 to be fed to the printing units 7a.about.7d will change considerably. That is, if the respective temperatures of the drag rollers 1, 2, and 3 increase, the respective diameters will increase. With this, the web transfer quantity is also increased and the state of the tension, deformation of elongation, etc. of the web 6 to be fed to the printing units 7a.about.7d changes considerably.
Thus, only the control of the respective speeds N of the drag rollers 1, 2, and 3 at the same ratio causes the transfer quantity of the web 6 to change. As a result, since the state of the web (such as tension, deformation of elongation, and the like) changes, there is a problem that the printing position will shift.
Particularly, at the transition time from start of operation to the temperature stability of the drag rollers 1, 2, and 3, the temperature changes of the drag rollers 1, 2, and 3 are conspicuous. Therefore, no matter how accurately the respective speeds N of the drag rollers 1, 2, and 3 are controlled, the printing position will continue to shift.